Such swivel joints are subjected to torsional stress and to a compressive stress in the direction of the pivotal axis of the joint and are often subjected to other stresses as well. These stresses give rise to problems in the design of such joints. Particularly when such joints are intended for artifical leg limbs, such stresses in addition to the usual torsional and compressive stresses may have a large magnitude, which often cannot be exactly ascertained numerically.
It is known from anatomy that a human being who is walking not only bends, stretches, abduces and adduces the legs but at the same time twists them about the mechanical longitudinal axis of the leg. In a normal, straight leg, this torsional axis or mechanical longitudinal axis extends from the center of the upper ankle joint through the center of the knee joint to the centre of the hip joint.
During walking, the legs are twisted most strongly in the hip joint and partly in the knee joint, but also in the joints of the foot. These movements, which can generally be described as twisting movements, are superimposed on opposite movements of the pelvis. The torsional axis is not normal to the surface which supports the leg but is inclined to that surface. As a result, the leg is not only twisted but a lateral inclination is imparted to the axis when the leg is stepping forward and when the foot is rolling on the ground. During the movement of the leg, the torsional axis thus describes a conical surface. The inclination is imparted to the axis about the upper ankle joint so that walking results in the known displacement of the center of gravity transversely to the direction of advance. The centre of gravity of the body of a person who is walking thus performs a sinusoidal movement with respect to the direction of advance.
After an amputation, the patient loses the ability to walk properly. He is then given training in which he learns to perform the described movement with the artificial limb, although this results in a rotation torsion between the soft envelope of the stump and the inside wall of the artificial limb in contact with said stump. This torsion gives rise to an irritation of the skin of the stump and to further inconveniences.
Various attempts have been made to design rotators which enable the patient to walk as naturally as possible and which avoid an irritation of the flesh of the stump (which may be a thigh stump or a tibia stump). Designs have been proposed in which the two relatively movable members bear on each other with interposed ball bearings while an elastomeric torsion spring exerts a restoring torque between the two members which are rotatable relative to each other. In such experimental swivel joints the elastomeric spring consists of an elastomeric ring, which is disposed between two end plates. The two end plates are secured to the two rotatable members fo the swivel joint. In another known design the axial gap between the relatively rotatable members is covered by a hoselike element, which is connected at its opposite ends to the relatively rotatable members by hose clamps. All these designs have the disadvantage that they are too heavy for practical use. Certain advantages for thigh-amputated patients have been afforded by such designs in experiments, particularly as regards the lessening of the irritation of the skin. However, the above-mentioned experimental designs do not appear to be suitable for tibia-amputated patients because they can bend the knee without difficulty when standing.